Photocatalysis belongs to the family of Advanced Oxidation Processes (AOP) that utilize an oxidant species to break carbon bonds with other carbon atoms, nitrogen, chlorine, sulfur, fluorine and other elements. The array of species that have been affected by photocatalysis in laboratory studies include, inter alia, simple organic compounds, chlorinated organic compounds, petroleum products, municipal wastewater, metal-containing photographic by-products, bacteria and viruses.
AOPs can either use an oxidant alone, or may be used in conjunction with a catalyst to promote its desired effect. Common stand-alone AOPs for the purpose of treating aqueous fluids are ozonation and combustion. Catalytic AOPs include hydrogen peroxide and a metal in the presence of ultraviolet (UV) light to promote hydroxyl radicals. This combination is commonly referred to as Fenton's Reagent. UV, at times, is considered an AOP. There are documented processes that utilize UV with ozone, or with hydrogen peroxide for the purpose of treating water and wastewater for organic destruction and disinfection.
Photocatalysis is an AOP based on a solid semiconductor material that is bombarded with UV radiation to excite the electrons and holes within the semiconductor material to produce oxidation-reduction (redox) reactions.
Two methods of photocatalysis have been suggested in literature. The first concerns the formation of free radicals. Electron-hole pairs migrate to the surface of the catalyst and react with hydroxyl ions (OH.cndot.) and dissolved oxygen (O.sub.2) to form hydroxyl radicals (OH.) in solution. Hydroxyl radicals then react with organic substrates in the fluid to oxidize them. Hydroxyl radicals have the highest oxidizing strength of common oxidizing species such as ozone, peroxide, and chlorine-based compounds.
The second method, a method most widely confirmed, is similar with electron, hole and hydroxyl reactions, but they take place on the catalyst surface with the absorbed organic species. As discussed, there are redox reactions taking place. At the anodic area (oxidizing) of the catalyst, holes are reacting with water to create hydroxyl radicals, and the organic species and their intermediate products. At the cathodic area (reducing) of the catalyst, the electrons are reacting with the oxygen to reduce it to the superoxide species, which in turn reacts with holes to assist in the organic matter oxidation. Precious metals that are metallized to the semiconductor (in areas not illuminated) aid in the reducing reactions at the cathodic area. It has also been shown in literature that precious metals act as oxidizers when in the illuminated area of the catalyst.
The art is often described in terms of either a suspended/slurried photocatalyst or a fixed photocatalyst. Suspended catalysts are those utilizing fine particles of a semiconductor material, generally to increase catalyst surface area. U.S. Pat. No. 5,589,678 (Butters, et al) provides a description of photocatalytic slurries. Suspended catalysts are limited to maximum concentrations in the fluid since they (1) increase turbidity, (2) absorb light, and (3) refract light, thus decreasing overall UV transmission in an illuminated reactor.
Fixed catalysts, to which the subject invention are directed, employ a singular or multi-pieced support or substrate to which the photocatalyst is applied. Fixed catalysts have been perceived as having less overall catalyst surface area then suspended catalysts, but do not require removal and recovery of the suspended catalyst particles. An example of a fixed catalyst support design is presented in U.S. Pat. No. 5,790,934 (Say et al). The Say invention utilizes multiple fins located in a radial or longitudinal arrangement and suffers from various shortcomings and limitations. First, the fixed substrate fins are situate at a certain distance away from the UV source. Reactivity is greatest in close proximity to the light source and decreases with distance. Also, the apparatus may not be inserted into existing UV chambers, nor allow for cleaning of the UV sources without removing the apparatus.
U.S. Pat. No. 5,126,111 (Al-Ekabi et al) provides a fiberglass mesh design, however, again it is located at a distance away from the UV source, cannot be inserted into commercial UV chambers, nor compress and expand to allow for UV source cleaning. Further, this invention requires the UV spectra to be in the range of 340-360 nm that is outside the capability of standard bulb designs, i.e. 185 nm and 254 nm. Other mesh designs are illustrated in U.S. Pat. No. 4,892,712 (Robertson et al) and U.S. Pat. No. 5,766,455 (Berman et al). Neither of these designs allow for close contact with the source or permit compression and expansion within a standard UV chamber.
Some fixed catalyst substrates have been proposed to increase overall catalyst surface area through catalyst absorption onto silica gel, zeolites, carbon black, and porous metals, however, the micropores of these fixed catalysts may not allow sufficient illumination to penetrate for efficient catalyst activation. Also, these materials are packed into a reactor where proper illumination of some surfaces of a majority of the catalysts may not be accomplished.
U.S. Pat. No. 5,501,801 (Zhang, et al) illustrates the use of silica gel and zeolite substrates as photocatalytic supports.
Another fixed substrate design is the use of titanium metal pieces (rods, spheres, beads, chunks, and the like) that are oxidized to form the desired titanium dioxide layer. As discussed in U.S. Pat. No. 5,868,924 (Nachtman, et al) and U.S. Pat. No. 5,395,552 (Melanson, et al), titanium metal, or its alloys, are inserted into a UV chamber along the length of the UV source, at a distance away from the UV source.
A replaceable coated cartridge is presented in U.S. Pat. No. 5,736,055 (Cooper) that provides a design for a replaceable piece in a photocatalytic reactor that combines a flexible photocatalytic surface with a rigid base. Again, the inner photocatalytic surfaces of the cartridge are at a distance away from the UV source(s) and are not readily adjustable to facilitate maintenance of the UV source(s).
Based on the above prior art, there has clearly been demonstrated an effort to enhance photocatalysis through, among other things, development of novel fixed-catalyst substrates. As will become apparent upon review of the detailed description below, Applicant has developed a new and improved fixed-catalyst substrate with several advantages heretofore unobserved.
Another means of enhancing photocatalytic reactivity involves the use of various oxidants, reducing agents and pH control agents. U.S. Pat. Nos. 5,126,111 (Al-Ekabi, et al), 5,779,912 (Gonzalez-Martin, et al), 5,863,491 (Wang), and 5,554,300 (Butters, et al) discuss the use of oxidants or reducing agents, or both, to promote photocatalytic reactions. Oxidizing and reducing agents act as hole and electron scavengers, respectively, to preclude electron-hole recombination that reduces photocatalytic efficiency. pH control agents are introduced to shift the reduction potential of the fluid to selectively oxidize/reduce targeted chemical species.
The method of injection of such agents into fluid treatment processes has been presented as general in nature, namely the injection into a fluid stream upstream of a reactor and the provision of sufficient motive force to allow mass transport to the catalyst surface. As previously mentioned, the photocatalytic reactions have been commonly observed to occur at the catalyst surface. It is therefore desirable to provide a photocatalytic reaction enhancement device which allows injection of an oxidant, reducing agent, or pH control agent, alone, or in combination with one and other, in direct proximity of the photocatalyst surface(s) for purposes of increasing fluid treatment efficiency. The subject invention is capable of enhancing photocatalytic reactivity through the employment of such means.